In order to be able to send and receive accurate information from the universe parabolic antennas must be aligned accurately. That means that the base on which the antennas – which weigh several thousand tons – turn must be held in constant position.
The core of the Deep Space Network is its compliment of 26m, 34m and 70m antennas.
The 70m antenna, which is referred to as a ‘sensitive ear’ by CDSCC workers, consists of a firm base and a mobile antenna structure. The mobile part – which weighs almost 4,000t – ‘swims’ on a 0.1mm thick film of oil, which makes accurate / even movements possible. This ensures that the DSS-43 can be positioned accurately to communicate with distant space probes millions of kilometres away.
The surface of the antenna base must be flat to an accuracy of 0.025mm. If position fluctuations occur, for example when the supporting base settles, shim-plates are used to adjust the level of the antenna. The shims are inserted between the fixed and the mobile part of the antenna to compensate the oil film thickness.
Originally the shims were manufactured from steel sheets and they were simply fixed to the grinding machine table with a magnetic chuck. "Over the last 30 years, though, the Americans have changed over to shim-plates made of fibreglass and reinforced composite material. As the metal shims corroded they had to be exchanged," says Bill Dobie.
However, this made machining composite sheets more difficult, because an alternative clamping method had to be found. First Dobie’s team fixed the composite shims with double-sided tape to the machine table with less than ideal results. About 50% of the produced shims were rejected as they did not comply with the required thickness specification. "It became obvious that vacuum clamping technology was our only alternative," Dobie stated.
The department of ‘antenna support’ has meanwhile worked successfully for several months with a vacuum grid plate supplied by Witte Far East, Singapore, which was manufactured to accommodate two workpieces simultaneously. The vacuum clamping range from Witte covers numerous varied systems. Depending upon type of machining and workpiece, suitable equipment is configured consisting of a vacuum chuck, vacuum pumps or units, and appropriate accessories.
Since this application concerns grinding fibreglass workpieces, using a vacuum grid plate seemed the obvious way to go. This is suitable for components with simple geometries and where heavy machining is involved, such as drilling, milling, and grinding.
The dimensions were chosen so that one shim can be clamped on its own or two shims simultaneously. Thus the vacuum grid plate was designed and manufactured to a size of 1205mm x 415mm. When only one shim-plate is needed, the clamping range on the chuck is delimited with an O-shaped seal. The vacuum supply runs through the slot grid. Only one suction hole is required for each workpiece.
During CDSCC’s application the workpieces are positioned on the vacuum chuck. The push of a button activates the vacuum pump, which sucks air out from underneath the workpiece thereby securely fixing the part for machining. The downforce is equivalent to between 800g/cm² and 1000g/cm², depending on height and atmospheric pressure. This means the fibreglass parts are held down with a force of approximately 1000kg.